This invention relates in general to an optical system for reading a video disc. More particularly, the invention is concerned with an improved focus error correction system for deriving a signal indicative of the extent and sense of departure of a reading light spot from registration with the storage track of a video disc.
In the field of video information storage and retrieval, the video disc record has been proposed as an adjunct to the conventional home color television receiver in order to augment its utility to the end that such a receiver can be employed as a playback device for pre-recorded video and audio program materials. A record of the type herein considered can comprise a vinyl disc having video or other information stored in a spiral track which physically can take the form of a train of pits and lands. Such a disc is described and claimed in copending patent application Ser. No. 439,680, filed Feb. 4, 1974, in the name of Adrianus Korpel, now U.S. Pat. No. 3,931,459, and assigned to the same assignee as the subject invention.
In order to retrieve the stored information, it is necessary to support the disc for rotation in a reading plane and to maintain a focused reading beam, desirably derived from a laser, in registration with the storage track. The maintaining of focus registration has, in the past, contemplated the derivation of an error signal, indicative of an out-of-focus condition, for use in a servo control system. Additionally, there has been resort to aerodynamic disc stabilizers to maintain such registration.
Insofar as the former is concerned, i.e., detection of an error signal, it is known to employ a cylinder lens for the purpose of developing astigmatic images and then to monitor the position of these images with a photodetector. The output signal of this detector is indicative of a displacement, in space, of these images which displacement, in turn, is indicative of an out-of-focus condition, that is, a departure of the record track from the reading plane.
However, experience with the aforementioned known arrangement has shown that the error signal derived in characterized by a undesirable shortcoming in that this signal is not linear as a function of the defocused condition. In other words, the extent to which the focus error signal changes, with respect to track departure from the reading plane in one direction, is not the same as the extent to which the signal changes when the track departs from the plane in the opposite direction. As a result, error signal capture of the focus servo control system is not symmetrical, i.e., the capture affect for one direction of defocusing is less expensive than the capture affect for the opposite direction of defocusing.